Oxygen reduction and/or oxygen evolution are key electrochemical reactions in fuel cells, rechargeable metal air batteries, water electrolysis systems, and chlor-alkali cells. A key component in these systems is the air electrode. Air electrodes may be designed to be bifunctional. The term bifunctional means that this air electrode may act as either as a cathode or an anode in a battery, fuel cell or other cells depending on which way electric current flows at any given time. Thus when acting as a cathode and accepting electrons from an outside source, it allows oxygen to be reduced to hydroxide ions. When acting as an anode, and electrons are delivered to external circuits, it oxidizes hydroxide ion in water to oxygen gas.
The following equations describe the reactions that may take place at a bifunctional electrode:Cathode: O2+2H2O+4e−<==>4OH—Anode: 4OH—<==>O2+2H2O+4e−
An air electrode, sometimes called a gas-diffusion electrode, is generally a porous, thin, lightweight, plate or sheet-like structure (typically about 0.05″ thick) that serves as a barrier to separate liquid electrolyte within an electrochemical system from surrounding, ambient air. One of the sides faces ambient air (oxygen) while the other side faces a liquid (generally aqueous) electrolyte. These two sides are deliberately designed to have differing water wetting properties and functions. The side facing ambient air is generally porous, air permeable, and deliberately made hydrophobic (water repelling). The opposite face—the side facing aqueous liquid electrolyte—is generally hydrophilic (water attracting). A current collector, sandwiched within/between these two regions, is designed to carry electrons to and from the air electrode. Characteristics of gas-diffusion electrodes may be strongly influenced by chemical and physical properties near/in its surface region. This invention disclosure describes ways of improving performance characteristics of bifunctional air electrodes.